The present invention relates generally to tunable capacitors and associated fabrication methods and, more particularly, to a hybrid microelectromechanical system (MEMS) tunable capacitor and associated fabrication methods.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. For example, one advantageous MEMS device is a variable capacitor in which the interelectrode spacing between a pair of electrodes is controllably varied in order to selectively vary the capacitance between the electrodes. In this regard, conventional MEMS variable capacitors include a pair of electrodes, one of which is typically disposed upon and fixed to the substrate and the other of which is typically carried by a movable actuator or driver. In accordance with MEMS technology, the movable actuator is typically formed by micromachining the substrate such that very small and very precisely defined actuators can be constructed.
While a variable or tunable capacitor can be utilized for many applications, tunable filters frequently utilize variable capacitors in order to appropriately tune the filter to allow or reject signals having predetermined frequencies, while, correspondingly, allowing or rejecting signals having other frequencies. For tunable filters that are utilized for high frequency applications, such as applications involving radio frequency (RF) signals, the tunable filter preferably has low signal loss and a high Q, i.e., a high quality factor. Unfortunately, variable capacitors that include electrodes formed of conventional metals generally do not have a sufficiently high Q for high frequency applications.
While electrodes formed of high temperature superconductor (HTS) materials would advantageously increase the Q of the resulting variable capacitor, the use of HTS materials is generally not compatible with the micromachining techniques, such as required to fabricate the actuator of a conventional MEMS variable capacitor. For example, the typical fabrication of a MEMS flexible membrane capacitor involves a release operation that provides for the membrane to be fabricated on a substrate and then subsequently released during processing. The release operation employs chemicals, i.e., etchants, that would likely damage the superconductor materials by altering their performance characteristics. Additionally, the flexible materials used to fabricate the flexible membrane do not possess the low loss characteristics necessary to render a high Q capacitor. For a more detailed discussion of MEMS flexible membrane capacitors, see U.S. patent application Ser. No. 09/464,010, entitled xe2x80x9cElectrostatically Controlled Variable Capacitorxe2x80x9d, filed on Dec. 15, 1999, in the name of inventor Goodwin-Johansson and assigned to MCNC, the assignee of the present invention. That application is herein incorporated by reference as if set forth fully herein.
As such, MEMS variable capacitors that have improved performance characteristics are desired for many applications. For example, tunable filters having a higher Q so as to be suitable for filtering high frequency signals are desirable, but are currently unavailable.
A tunable capacitor is therefore provided that is micromachined so as to be precisely defined, extremely small and provide microelectromechanical actuation. In one embodiment the capacitor plates are formed of a high-temperature superconductor (FTS) material. As such the tunable capacitor can be utilized for a wide variety of high performance applications having a high Q requirement. For example, a tunable filter using a tunable high Q capacitor and inductor can appropriately filter high frequency signals, such as radio frequency (rf) signals.
The MEMS tunable capacitor includes a first substrate having a first capacitor plate disposed thereon. A fixed pivot structure is typically disposed on the first substrate, proximate the first capacitor plate. The fixed pivot structure can serve as a point of attachment for a flexible membrane that extends outward from the fixed pivot and generally overlies the first capacitor plate. The MEMS tunable capacitor also includes a flexible membrane and a second capacitor plate carried thereby, typically upon a second substrate mounted to the underside of the flexible membrane, the flexible membrane overlies the first substrate such that the first and second capacitor plates face one another in a spaced apart relationship. A MEMS actuator is operably in contact with the flexible membrane for the purpose of providing an actuation force to the flexible membrane, thereby varying the capacitance between the first and second capacitor plates.
In one advantageous embodiment of the invention the first and second capacitor plates comprise an HTS material and the first and second substrates may comprise a low signal loss material that is compatible with the HTS material. In one embodiment of the invention, the MEMS actuator that is used to provide actuation to the flexible membrane is a MEMS electrostatic flexible film actuator. The flexible film actuator is attached to the first substrate and operably contacts the flexible membrane, typically, at a point furthest from the fixed pivot structure. The flexible film actuator will typically include a third substrate attached to the first substrate and having a substrate electrode disposed thereon. Additionally, a flexible film composite overlies the third substrate and includes an electrode element and at least one biasing element. In lengthwise definition, the flexible film will comprise a fixed portion attached to the underlying third substrate and a distal portion extending from the fixed portion and generally overlying the substrate electrode. An insulator will typically be disposed between the substrate electrode and the flexible film electrode to provide electrical isolation. The flexible film composite is actuated in response to the application of electrostatic force that is applied between the substrate electrode and the flexible film electrode. In response to the actuation of the flexible film, the operably contacted flexible membrane of the tunable capacitor will positionally change and thus vary the capacitance between the first capacitor plate and the second capacitor plate.
In additional embodiments of the invention the MEMS actuator used to provide actuation to the operably contacting flexible membrane may comprise a MEMS thermal arched beam actuator, a MEMS thermal bimorph actuator, a MEMS piezoelectric actuator or any other MEMS actuation means.
Additionally, the present invention is embodied in a method for making a tunable capacitor. The method comprises fabricating a first capacitor plate construct formed of a first substrate having a first capacitor plate disposed thereon, the first capacitor plate, typically, comprising a HTS material. Additionally, the method entails fabricating a MEMS actuator that is responsive to actuation forces and attaching the MEMS actuator to the first substrate proximate the first capacitor plate. A second capacitor plate structure is fabricated that comprises a flexible membrane, a second substrate attached to the flexible membrane, and a second capacitor plate disposed on the second substrate. The second capacitor plate will typically comprise a HTS material. The tunable capacitor is completed by connecting the first capacitor plate structure to the second capacitor plate structure via a fixed pivot structure that allows the first and second capacitor plates to generally face each other in a spaced apart relationship and provides for the flexible membrane of the second capacitor plate structure to be in operable contact with the MEMS actuator.
According to the present invention, a tunable capacitor and an associated fabrication method are provided which permit micromachining techniques to be used to fabricate a tunable capacitor actuated by MEMS actuators. The MEMS actuators may include electrostatic flexible film actuators, thermal bimorph actuators, thermal arched beam actuators, piezoelectric actuators and the like. In one advantageous embodiment the tunable capacitor plates are formed of a high temperature, super conductor material. As such, the tunable capacitor can be precisely defined, small in size and MEMS actuated, while also having improved performance characteristics relative to conventional tunable capacitors. Thus, the tunable capacitors of the present invention can be used in a variety of applications, including those requiring high Q, such as, filtering signals having high frequencies.